84 J. Thos. Patterson. 



the more superficial ones. He says, ''in the part where differentia- 

 tion is going on, the cells of the outer row and those of the inner 

 rows are found pushing between one another, and the layer of such 

 condition passes over gradually into the part which has already 

 become a true epithelium."^-'' The expansion of the upper hemisphere 

 necessarily brought about by this differentiation plays an important 

 role in gastrulation. 



Hatta points out the homology existing between the blastulation 

 of Amphioxus and the differentiation of the micromeric layer into 

 an epithelium in Petromyzon. He contends that since it is incorrect 

 to speak of a "blastula" stage in Amphioxus before the blastomeres 

 ''are converted into the form of an epithelium," so in Petromyzon 

 it is correct to speak of blastulation only when differentiation of 

 blastomeres into an epithelium has begun. In regard to the latter 

 form he writes, "In this case blastulation, as indicated by differentia- 

 tion of the blastomeres into an epithelium, should be looked upon 

 as being much delayed; it is still being carried on during the whole 

 period of the gastrulation and is finished only a little earlier than 

 the latter process. In other words, the two processes, blastulation 

 and gastrulation, overlap each other to a great extent in the period 

 of their occurrence. The prime cause of this belated mode of 

 development is indisputably due to delay of segmentation on account 

 of an enormous accumulation of yolk within the ovum."^'^ 



Without referring to the various eggs showing intermediate con- 

 ditions, we may consider next the thinning-out in an egg in which 

 the accumulation of yolk within the ovum is carried almost to the 

 extreme, that is, in a meroblastic egg such as that of the Selachian 

 Torpedo (Zieglers, '92), or Pristiurus (Riickert, '99). In Torpedo 

 Ziegler figures and describes a "blastula" stage in which the posterior 

 portion of the blastodisc is differentiated into a single-layered 

 epithelium, while anteriorly it gradually increases in depth and 

 the cells are not arranged in the form of an epithelium. At this 

 stage, invagination of the thin posterior margin begins and soon 

 after, the differentiation (thinning-out) extending both anteriorly 



^^Loc. cit., p. 24. 

 "Loc. cit., p. 35. 



